As part of our larger interest in understanding the molecular and cellular events that govern initiating events in the immune response, we have undertaken to develop a system in which molecular biological approaches can be combined with evolutionary biology and developmental genetics to explore not only how the immune system functions in advanced organisms but also how it develops and functions in evolutionarily more primitive species. Because of the unique advantages of the potential for study of both phylogeny and ontogenetic development in the same organism, we have undertaken to isolate the genetic and biochemical components of the immune system of the zebrafish, Brachydanio rerio. In this project, we have isolated cDNAs for the major histocompatibility complex (MHC) class I molecule Brre-UBA*01, for the MHC class I light chain, beta2-microglobulin, and have identified a number of candidate genes encoding either T cell receptor constant regions or immunoglobulin light chain constant regions. We have expressed both an MHC class I heavy chain and beta2-microglobulin in bacteria, have refolded both these proteins efficiently, and are in the process of evaluating these proteins in detail. We expect that this battery of molecular biological and protein reagents will allow us to further evaluate how the function of MHC/TCR recognition evolved in the several hundred million years that separate the origin of the ancestors of the teleost fish (of which B. rerio serves as an example) and those of more advanced vertebrates.